docs: def tensor_index(self, datapoint_index):
          """ Returns the index of the tensor containing the referenced datapoint. """
          if datapoint_index >= self._num_datapoints:
              raise ValueError('Datapoint index %d is greater than the number of datapoints (%d)' %(datapoint_index, self._num_datapoints))
          return self._index_to_file_num[datapoint_index]